Many of single-lens reflex cameras perform auto focusing (AF) by the TTL (Through the Taking Lens) phase difference detection method. In the phase difference detection method, the luminous flux from an object taken by the taking lens is reflected by a movable mirror and led to a plane (primary imaging plane) equivalent to the light-receiving surface of a film or an image-pickup device. The luminous flux led to the equivalent plane is divided into two luminous fluxes (pupil division) by a secondary imaging optical system including a separator lens, and the two luminous fluxes are led to a pair of line sensors for AF. Then, the displacement of two images on the pair of line sensors, that is, a phase difference is detected, and thereby the displacement of the focus lens in the taking lens from an in-focus position can be obtained.
On the other hand, since the digital camera has an image-pickup device performing the photoelectric conversion of object image, the line sensors dedicated to AF and the secondary imaging optical system separated from the taking lens are not required if part of the image-pickup device is used as the AF sensor area.
Accordingly, a focus detection system has been disclosed in Japanese Patent Laid-Open No. 2004-46132 and, Aizo Suzuki, “Camera Technology Handbook”, pp. 108-111, Shashin Kogyo Publishing, Jul. 15, 1979, in which part of the image-pickup device is used as AF sensor areas, and the luminous fluxes divided by an image splitting prism provided in the taking optical system are led to the areas.
Further, an AF system for the TTL phase difference detection method has been disclosed in Japanese Patent Laid-Open No. H04-147207, in which a holographic optical element is arranged on a side closer to the object than the primary imaging plane.
However, in the focus detection system disclosed in Japanese Patent Laid-Open No. 2004-46132, an object image larger than the image splitting prism is required unlike in the case where the TTL phase difference detection method is used. Further, there are some restrictions in this focus detection system. For example, if the shape of the image on the boundary of the image splitting prism is not linear, the system determines that the lens is in an out-of-focus state thought it is in an in-focus state. Therefore, it is not possible to obtain a focus detection performance equivalent to that of the TTL phase difference detection method.
Moreover, the method disclosed in Japanese Patent Laid-Open No. H04-147207, in which the holographic optical element is used, is the same as the TTL phase difference detection method in principle. However, the method uses the holographic optical element having a large chromatic dispersion for producing two images (phase difference images) in the direction of the pupil division, images which are important for the determination of the focus state. Therefore, the difference of the incident angles between the phase difference images, which is important for obtaining sufficient focus detection accuracy, changes greatly depending on wavelengths. Accordingly, the method is unrealistic to the focus detection.